1. Field of the Invention
The present invention relates to a method of measuring optical attenuation of an optical fiber cable including an optical branching waveguide using an optical time domain reflectometer.
2. Background Art
An optical time domain reflectometer (OTDR) is used to detect a point which is damaged, or to measure an energy loss, such as a transmission loss or a connection loss in an optical fiber cable by outputting a light pulse from the OTDR to a measuring optical fiber cable through an optical waveguide directional coupler and by detecting a returned light pulse from the measuring optical fiber cable.
FIG. 4 is a block diagram showing a measurement system for a conventional method of measuring optical attenuation. In FIG. 4, a schematic diagram of a measuring optical fiber cable 5 consisting of an optical branching waveguide 7, an optical fiber cable 6 at the supplying side, and optical fiber cables 8, 9, 10, . . . , is shown. The optical fiber cable 6 has a common input terminal and a plurality of output terminals. One end of the optical fiber cable 6 is connected to the common supplying terminal of the optical branching waveguide 7, and one end of each optical fiber cable 8, 9, 10, . . . is connected to each output terminal of the optical branching waveguide 7.
In a conventional method, an operation to measure attenuation is carried out by connecting an OTDR 1 to the optical fiber cable 6 which is a common optical fiber of the measuring optical fiber cable 5. In this case, it is necessary to secure an extremely large dynamic range of the OTDR 1, as described in "FIBER MEASUREMENT TECHNIQUES FOR PASSIVE DOUBLE STAR NETWORKS", Third IEEE Workshop on Local Optical Networks; 1991, Vol. 9, pp. 24-25.
As described in the above-mentioned reference, for example, when the number of branches of the optical branching waveguide 7 is 32, the dynamic range of the OTDR 1 must be approximate 32 dB without energy loss from the optical fiber, and this is not practical.
Furthermore, another method of measuring optical attenuation uses an optical power meter and an optical light source as a measurement apparatus. However, each measuring apparatus must be placed at an end of the optical fiber cable, respectively. In addition, each optical fiber must be measured along every branch. As a result, the measurement operations are troublesome. Furthermore, in the case of a line which is in service, the measuring apparatuses must be placed at the ends of all branches, and this is not practical.